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121 lines
3.4 KiB
Markdown
121 lines
3.4 KiB
Markdown
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[← Home](../../README.md) · [Hardware](../README.md) · [AGA](README.md)
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# AGA Chipset Internals — Alice & Lisa
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## Alice (MOS 8374) — AGA Agnus
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Alice is the successor to Super Agnus and is the DMA controller and Copper/Blitter engine for AGA machines.
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### Key Enhancements over Super Agnus
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**64-bit DMA fetch bus (FMODE):**
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Alice can fetch 2 or 4 words per DMA cycle via the `FMODE` register ($DFF1FC). This dramatically increases the bandwidth available to the blitter and bitplane DMA.
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**Extended bitplane depth:**
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Alice supports up to **8 bitplanes** (256 colours), compared to OCS/ECS's 6-plane limit.
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**BPLCON4:**
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Alice adds `BPLCON4` to control bitplane bank selection — which 64-entry block of the 256-entry colour table is used by the bitplanes.
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### ALICE_ID
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Alice can be identified via `VPOSR`:
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```asm
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move.w $DFF004, d0 ; VPOSR
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lsr.w #8, d0
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```
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| VPOSR[15:8] | Chip |
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|---|---|
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| $22 | Alice AGA (standard) |
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| $23 | Alice AGA (some A4000 revisions) |
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---
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## Lisa (AGA Denise)
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Lisa is the display chip successor to ECS Denise, providing 8-bit colour output (256 colour registers) and extended sprite capabilities.
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### Key Enhancements over ECS Denise
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**256 colour registers:**
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Lisa provides COLOR00–COLOR255, each 24-bit (32-bit register with low byte unused).
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**4 colour banks for bitplanes:**
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`BPLCON4` selects which 64-register bank (0–3) the bitplanes use for lookup. This allows dual-playfield each using a different 64-colour palette.
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**Sprite bank selection:**
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`BPLCON3` bits select which colour bank sprite pairs use.
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**Extended sprite width:**
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Sprites can be 16 or 64 pixels wide in AGA mode.
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**Lisa ID:**
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Readable from `$DFF07C` (DENISEID):
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```asm
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move.w $DFF07C, d0 ; DENISEID = $00F8 for AGA Lisa
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```
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---
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## FMODE — DMA Fetch Width Register ($DFF1FC)
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The most critical AGA-specific register. Controls the data bus width for blitter and bitplane DMA:
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```
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bits 15-14: SPR_FMODE — sprite fetch mode
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bits 13-12: BPL_FMODE — bitplane fetch mode
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bits 9-8: BLT_FMODE — blitter fetch mode
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00 = 1× (16-bit, OCS/ECS compatible)
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01 = 2× (32-bit)
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10 = 4× (64-bit)
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11 = reserved
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```
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**Setting full 64-bit blitter mode:**
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```asm
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move.w #$00C0, $DFF1FC ; FMODE: BLT_FMODE=11 (64-bit)
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; Also set BPL and SPR modes as needed
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```
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> [!CAUTION]
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> Writing FMODE on OCS/ECS machines writes to the `LISAID` location (read-only) — no hardware damage, but the read-back value is incorrect. Always verify AGA presence before writing FMODE.
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---
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## DMA Bandwidth with FMODE
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| FMODE | Bus width | Blitter speed | Bitplane DMA |
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|---|---|---|---|
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| 1× (OCS compat) | 16-bit | 1× | 1× |
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| 2× | 32-bit | 2× | 2× |
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| 4× | 64-bit | 4× | 4× |
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At 4× mode, the AGA blitter can fill/copy at ~70 MB/s theoretical on a 7 MHz bus.
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---
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## BPLCON0 Extended Bits (AGA)
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In AGA mode, `BPLCON0` bit 4 (`ECSENA`) must be **1** to enable AGA features. Additional BPU bit (bit 4 of the count) allows 7 and 8 planes:
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```
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bits 14-12: BPU2-0 — lower 3 bits of bitplane count
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bit 4: BPU3 — MSB of bitplane count (AGA: allows 7, 8 planes)
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```
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To use 8 bitplanes (256 colours):
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```asm
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move.w #$9411, BPLCON0+custom ; HIRES=1 (if needed), BPU=8 (BPU3=1, BPU2-0=000), ECSENA=1
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```
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---
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## References
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- ADCD 2.1 Hardware Manual — AGA chapter
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- NDK39: `hardware/custom.h` — struct Custom (with AGA extensions)
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- Commodore A1200 Technical Reference Manual — Alice/Lisa section
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- AmigaMail Vol. 2 — AGA programming articles
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